staging: rtl8712: Remove r8712_setassocsta_cmd()

Remove function r8712_setassocsta_cmd as it is only called by
mp_start_joinbss, which was removed in the previous patch.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>